1. Field of the Invention
The present invention relates to an electrostatic capacitive type ultrasound transducer element and an ultrasound endoscope including the ultrasound transducer element.
2. Description of the Related Art
An ultrasound diagnostic method that irradiates an inside of a body with ultrasound, and images a state of the inside of the body from an echo signal to perform diagnosis is widespread. One of the ultrasound diagnostic apparatuses for use in the ultrasound diagnostic method is an ultrasound endoscope (hereinafter, called “a US endoscope”). In a US endoscope, an ultrasound transducer is placed at a distal end rigid portion of an insertion portion that is introduced into an inside of a body. An ultrasound transducer has a function of converting an electric signal into ultrasound and transmitting the ultrasound to the inside of a body, and receiving ultrasound reflected in the inside of the body and converting the ultrasound into an electric signal.
For an ultrasound transducer, a ceramic piezoelectric material containing lead that has large environmental load, for example, PZT (lead zirconate titanate) is mainly used. In contrast with this, development of an electrostatic capacitance type ultrasonic transducer (capacitive micro-machined ultrasonic transducer; hereinafter, called “a c-MUT”) that does not contain lead in the material and is produced with use of MEMS (micro electro mechanical systems) technique is being advanced.
As is disclosed in Japanese Patent Application Laid-Open Publication No. 2005-510264, a c-MUT has an ultrasound cell (hereinafter, called “a US cell”) in which an upper electrode portion and a lower electrode portion are disposed to face each other via a cavity portion (cavity) as a unit element. In the US cell, a membrane including the upper electrode portion on an upper side of the cavity configures a vibration portion. A plurality of US cells with the respective electrode portions connected by a wiring portion are arranged to configure an ultrasound transducer element (hereinafter, called “a US element”).
The US cell vibrates the membrane including the upper electrode portion by an electrostatic force by application of a voltage to between the lower electrode portion and the upper electrode portion to generate ultrasound. Further, when ultrasound is incident on the membrane from an outside, the membrane deforms to change a spacing of both the electrode portions, and therefore, the ultrasound is converted into an electric signal from a change in electrostatic capacitance.